The invention relates to a process for output control and limitation in a heating surface made from glass ceramic or a comparable material, especially a glass ceramic cooking surface, in which the individual heating zones of the heating surface are heated in a way known in the art with heating devices with several heating elements which are switchable and controllable independently of one another. The invention also relates to a preferred device for performing the process in a cooking area with a glass ceramic cooking surface.
Heating surfaces made from glass ceramic or a comparable material are also used, for example, as wall or ceiling radiators, heat exchangers, or other large-surface heating devices, which can be heated in any way.
Electrically or gas-heated cooking areas or individual burners, whose heating surface consists of glass ceramic, are now of special interest. Cooking areas of this type are generally known and have already been described many times in the patent literature. Heating of the heating zones of these cooking areas (without narrowing the concept, the heating zones in the cooking areas below are also named cooking zones) takes place by heating devices, e.g, electrically operated contact heating elements, radiant heating elements or gas burners, placed below the glass ceramic cooking surface. Further, induction cooking areas are also known.
In the known household cooking areas, the heat output for the heating devices is permanently adjusted by the presetting of the user or electronically, electromechanically or, with gas stoves by valves, purely mechanically controlled by a selectable time program. Corresponding controls are described, for example, in patent specification DE-PS 3 639 186 A1.
It is known to heat heating zones of a glass ceramic cooking area, which exhibit a sizable diameter, for example, to heat pots with sizable diameter and/or nonround, for example, oval, bottom surfaces with heating elements with several heating circuits. It is also known to use, besides the permanent heating elements constantly in operation, so-called auxiliary heating elements, which are actuated only in the boiling phase, to achieve an accelerated heating-up of the cooking zone. In this case, the geometric arrangement of the heating elements or heating circuits below a heating zone then is usually matched to the geometry of the cookware.
Thus, for example, a hot plate with two heating circuits, concentric to one another, is described in DE-OS 33 14 501 A1, in which the outside heating circuit is designed as an auxiliary heating circuit.
DE-PS 34 06 604 relates to a heating device, in which the heating zone is heated by several high-temperature and normal-temperature radiant heating elements. The heating elements in this case are placed so that the heating point is divided into two zones, concentric to one another, and the inside zone can be heated only by the high-temperature radiant heating elements usable preferably as auxiliary heating elements in the boiling phase and the outside zone by the normal-temperature radiant heating elements. A comparable arrangement of several radiant heating elements in the area of a cooking zone is also to be found in U.S. Pat. No. 4,639,579.
A heating device with a gas burner, which exhibits two burner chambers, independent of one another and able to be actuated with gas, which, e.g., can delimit zones, concentric to one another, in the cooking zone area, is described in U.S. Pat. No. 4,083,355.
In the glass ceramics usually used, the maximum operating temperatures are to be limited to 700.degree. C. To avoid overheating the glass ceramic heating surface, therefore as a rule so-called protective temperature limitation devices, e.g., a bar expansion switch placed mostly along a diameter between the heating elements and the glass ceramic surface, are used, which usually turn off the heating device completely or reduce its output when a specific temperature limit is exceeded. After passing through a hysteresis, the full heat output is again turned on. A bar expansion switch, for example, with two different switch points, which operates accordingly at two different temperatures, is known from DE-OS 3 314 501.
From German patent specification DE-PS 21 39 828, it is known that glass, glass ceramic or similar materials have an electrical resistance dependent on the temperature, so that temperature-measuring resistances with steep resistance-temperature characteristics, similar to the known NTC resistances, can be produced from them by applying strip conductors, e.g., made from noble metals.
This type of temperature sensors is used in DE-OS 37 44 372, in connection with the corresponding wiring, to replace the above-mentioned protective temperature limitation device completely. For this purpose, in each cooking zone in each case, two strip conductors, parallel to one another, which each delimit a strip-like glass ceramic resistance, are applied along a half diameter on the glass ceramic cooking surface.
Experience has shown that anomalous thermal stress conditions in glass ceramic cooking surfaces result mostly from using inferior cookware or operating errors.
Thus, e.g., in cookware with uneven support surfaces, a locally varying removal of heat takes place in the cooking zone. By carelessness, empty cookware can cause still higher temperature/time stresses for the glass ceramic. Pots with too small diameters as well as those inadvertently placed, i.e., pots which are not centered, cause additional extreme stresses. In these cases, the cooking zone in the areas not covered by the pot is overheated. The surface temperature of the glass ceramic can in such cases be considerably above the temperatures measured in the potless operation. Temperature increases of up to 200 K above the surface temperature in the potless operation are possible.
These anomalous thermal stresses in the area of the cooking zones can add up to high temperature/time stresses over time and can bring about the destruction of the cooking surfaces. Extremely high temperatures can damage the surface-mounted cookware and also the glass ceramic cooking surface. Pot enamel can, for example, melt in the case of steel enamel cookware which is inadvertently placed empty on a glass ceramic cooking surface. Also, aluminum cookware left on the cooking surface while empty can damage the glass ceramic surface by melting aluminum.
Since, in practice, both inferior or unsuitable cookware is used and the above-mentioned operating errors occur, the maximum surface temperature in the potless operation has to be limited. For the same reason, the specific output density of the heating devices, relative to the surface of the heated zone, is now limited to about 7 watt/cm.sup.2.
The above-described anomalous stress conditions, on the one hand, can lead to damage of the glass ceramic cooking surface and, on the other hand, considerably worsen the efficiency of the cooking system.
It is known that with inferior cookware, the average output offered by the heating device can be increased if the potless operation adjustment of the heating device is increased. This generally leads to a shortening of the boiling time. But with the constant use of this 10 cookware, exceeding the stress limits of temperature/time and thus the possible destruction of the glass ceramic cooking surface, cannot be eliminated by the increase of the potless operation adjustment.
With the use of good cookware, no increase of the average output can be achieved with this method, and connected with it, the boiling time be lowered. Good cookware withdraws so much heat from the glass ceramic that the protective temperature limitation device responds rarely or not at all during the boiling processes. The full nominal output of the heating device is generally always available in boiling processes in connection with good cookware. The efficiency can be increased here only by raising the heat output and by simultaneously raising the potless operation adjustment of the protective temperature limitation device with the drawbacks already described.